When microcomputers were introduced, as little as 16 Kbytes of memory resided in the central processing units. Eventually, a fully expanded 8-bit processor offered 64 Kbytes of addressable memory, which was very adequate for users needs. The subsequent development of personal computers resulted in computers having typically ten times the system memory of the foregoing microcomputers and employing a powerful disk operating system (DOS) which was quickly recognized as the standard of the industry. One of the basic limitations of DOS is that it operates within the 1 megabyte limit with no capability for utilizing memory beyond this limit. In order to solve the problems associated with this memory limitation, expanded memory specification (EMS) was developed. The 1 megabyte address space was divided into random access memory from 0 to 640 Kbytes leaving 384 Kbytes of memory remaining. This 384 Kbyte portion of memory was divided into six equal portions of 64 Kbytes of memory each referred to as A, B, C, D, E and F for their hexadecimal representations. These portions of memory are typically used for various purposes. For example, portions E and F are reserved for BIOS ROM memory, while video adapters and other interface adapters are placed into other portions. Expanded memory specification (EMS) utilizes three of these portions, i.e., portions C, D, and E. Basically, EMS translates an address below 1 Mbyte into memory above 1 Mbyte, thus bypassing the foregoing DOS limitation. A problem arises, however, in that the prior art approaches for accessing and decoding EMS bankselect signals have inherent time delays resulting in the system operating at a lower clock frequency. In those cases where the accessing and decoding operations could not be accomplished rapidly enough, the prior art solution was to disallow EMS at fast clock speeds or to force start delays.
Because of the inherent time delays associated with prior art approaches for accessing and decoding EMS signals, it has become desirable to develop a system for decoding bankselect signals in EMS systems which minimizes such delays and which can be operated at high frequencies, such as 33 megahertz, within a complex memory map structure.